Treatment of phosphate-containing wastewater

ABSTRACT

A method for treating, and recovering phosphate compounds from, phosphate-containing wastewater. The method includes the steps of: (a) removing fluoride from the wastewater; (b) recovering a phosphate compound from the wastewater by maintaining super saturation conditions for the phosphate compound; and (c) polishing the wastewater. A silica removal step may be optionally performed after step (a) and before step (b).

This application claims the benefit under 35 U.S.C. § 119 of U.S.provisional patent application No. 61/346,002, which is herebyincorporated herein by reference.

TECHNICAL FIELD

The invention relates to treatment of phosphate-containing wastewater,such as phosphogypsum pond water, and the recovery of useful phosphatecompounds, such as struvite, during treatment.

BACKGROUND

Phosphogypsum is a by-product of processing phosphate rock intophosphoric acid fertilizer. The production of 1 ton of phosphoric acidgenerates approximately 4 to 5 tonnes of phosphogypsum. Phosphogypsum isessentially a waste product. Phosphogypsum may have low levelradioactivity which prevents its use in various applications.

Phosphogypsum is typically stored by being slurried and piled into largestacks, which can be up to hundreds of feet high, in open air storagesites. Water percolating through the stacks forms ponds. In 2005, therewere 24 phosphogypsum stacks in Florida alone, containing 1.2 billiontonnes of phosphogypsum and 50 billion gallons of pond water (Perpich etal, 2005).

In active phosphoric acid fertilizer plants, such ponds are typicallyused as reservoirs for process water for use in a closed loop. The pondwater is toxic and needs to be treated before it can be discharged.Furthermore, closed stacks continue to produce a contaminant-containingleachate requiring treatment.

Pond water associated with phosphogypsum stacks is strongly acidic andcontains numerous contaminants including large amounts of phosphates.Data collected from a number of sources are summarized in Table 1. Thecolumn headed “Representative Value” contains results from a compositeof 18 samples from 6 different plants representing the composition ofsaturated fresh pond water (Kennedy et al., 1991). The reportedphosphorus concentration of 6,600 ppm as P is equivalent to 20,220 ppmPO₄ or 0.22 Mol/L. Pond water also contains significant amounts ofammonia (ammonia is often added to phosphoric acid in phosphoric acidplants to make di-ammonium phosphate) and magnesium.

TABLE 1 Typical composition of pond water Component Units RangeRepresentative Value pH 1.3-3.0 1.55 Conductivity μS/cm 15,000-30,000Ammonia (as N) ppm  500-2,000 592 Calcium ppm  500-3,000 1,155 Chlorideppm  10-300 Fluoride ppm   200-15,000 7,600 Iron ppm  5-300 216Magnesium ppm 200-500 286 Phosphorus (as P) ppm   500-12,000 6,600Potassium ppm 100-400 276 Silica (as Si) ppm  100-4,000 1,910 Sodium ppm1,000-3,000 1,995 Sulfate (as S) ppm 1,000-4,000 1,695

Pond water treatment chemistry is relatively complex. Pond water maycontain ten major components that can form numerous soluble species andprecipitates when the pH is changed and cations are added. As indicatedby the data under the column headed “Range” in Table 1, the compositionof pond water can vary significantly.

A cost-effective and efficient process for treating phosphate-containingwastewater while recovering commercially useful phosphate compoundswould be desirable.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with apparatus and methods which are meant tobe exemplary and illustrative, not limiting in scope. In variousembodiments, one or more of the above-described problems have beenreduced or eliminated, while other embodiments are directed to otherimprovements.

In one aspect, a method for treating, and recovering phosphate compoundsfrom, phosphate-containing wastewater, is provided. The methodcomprising: (a) removing fluoride from the wastewater; (b) recovering aphosphate compound from the wastewater by maintaining supersaturationconditions for the phosphate compound; and (c) polishing the wastewater.

Step (a) may comprise precipitating the fluoride. Precipitating thefluoride may comprise raising the pH of the wastewater to about pH 3 to4. Raising the pH of the wastewater may comprise adding acalcium-containing base with calcium in a quantity to meetstoichiometric requirements for precipitating the fluoride. Thecalcium-containing base may be lime. The fluoride may be precipitated asfluorite. Raising the pH of the wastewater further may comprise addingone or more calcium-free bases with cations in a quantity to meetstoichiometric requirements for precipitating the phosphate compoundafter measuring the concentration of the phosphate compound's precursorions. The one or more calcium-free bases may be selected from the groupconsisting of magnesium oxide, magnesium hydroxide, ammonium hydroxideand anhydrous ammonia.

The phosphate compound may comprise struvite or a struvite analog suchas iron ammonium phosphate.

Maintaining supersaturation conditions in step (b) may comprise one ormore of: maintaining a supersaturation ratio of 2 to 5; maintaining a pHof at least about pH 5; controllably introducing magnesium and/orammonium; and maintaining a concentration of phosphate higher thanconcentrations of magnesium and ammonia. The struvite may be recoveredin the form of crystals and aggregates ranging in size from 1 to 5 mm.

Silica may be removed from the wastewater of step (a) if the wastewaterfrom step (a) comprises a silica concentration of greater than 100 ppm.Removing silica may comprise hydrolyzing the silica by raising the pH.Raising the pH to hydrolize the silica may comprise adding a basecomprising cations in a quantity to meet stoichiometric requirements forprecipitating the phosphate compound after measuring the concentrationof the phosphate compound's precursor ions. The base may be selectedfrom the group consisting of magnesium oxide, magnesium hydroxide,ammonium hydroxide and anhydrous ammonia.

Polishing step (c) may comprise raising the pH to about pH 8 to 10. Step(c) may comprise removing ammonia using a process selected from thegroup consisting of breakpoint chlorination, stripping, biologicalnitrification and biological denitrification.

Polishing step (c) may comprise subjecting the wastewater from step (b)to a two-stage membrane treatment comprising: (i) a first membranetreatment to obtain a first concentrate comprising divalent ions and afirst permeate comprising monovalent ions; and (ii) a second membranetreatment for the first permeate to obtain a second concentratecomprising monovalent ions and a second permeate comprising effluent.The first concentrate may be recirculated to step (a). The firstmembrane treatment may comprise nanofiltration. The second membranetreatment may comprise reverse osmosis. Prior to the two-stage membranetreatment, the pH may be lowered to about pH 3 to 5, and suspendedsolids may be removed by filtration. Ammonia may be removed from thesecond permeate by subjecting the second permeate to ion exchange.Ammonia-containing regeneration liquid of the ion exchange may berecirculated to step (b).

Prior to step (b) the wastewater may be subjected to a first membranetreatment to obtain a first concentrate comprising divalent ions and afirst permeate comprising monovalent ions, wherein the first concentratedefines feed for step (b). Wastewater from step (b) may be recirculatedto step (a). The first permeate may be subjected to a second membranetreatment to obtain a second concentrate comprising monovalent ions anda second permeate comprising effluent. The first membrane treatment maycomprise nanofiltration. The second membrane treatment may comprisereverse osmosis. Prior to the two-stage membrane treatment, the pH maybe lowered to about pH 3 to 5, and suspended solids may be removed byfiltration. Ammonia may be removed from the second permeate bysubjecting the second permeate to ion exchange. Ammonia-containingregeneration liquid of the ion exchange may be recirculated to step (b).

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate non-limiting embodiments of theinvention.

FIG. 1 is a flowchart illustrating a process for treatingphosphate-containing wastewater according to one embodiment of thepresent invention.

FIG. 2 is a block diagram illustrating a process for treatingphosphate-containing wastewater according to another embodiment of thepresent invention.

FIG. 3 is a flowchart illustrating a process for treatingphosphate-containing wastewater according to one embodiment of thepresent invention.

FIG. 4 is a block diagram illustrating a process for treatingphosphate-containing wastewater according to another embodiment of thepresent invention.

FIG. 5 is a flowchart illustrating a process for treatingphosphate-containing wastewater according to one embodiment of thepresent invention.

FIG. 6 is a block diagram illustrating a process for treatingphosphate-containing wastewater according to another embodiment of thepresent invention.

DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well-known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

Some embodiments of the invention relate to methods for treatingphosphate-containing wastewater while simultaneously recoveringcommercially useful phosphate compounds. Bases are used to neutralizethe acidity of phosphate-containing wastewater. Cations from the basesare used to remove contaminants and recover phosphate compounds. Excesscations may be recirculated to maximize contaminant removal and recoveryof phosphate compounds.

Some embodiments of the invention relate to treatment processes whereinthe phosphate-containing wastewater is phosphogypsum pond water and thephosphate compound is recovered in the form of granular struvite. Theseembodiments coincide with an aspect of the invention having significantcommercial utility. The scope of the invention, however, is not limitedto these embodiments.

FIG. 1 illustrates in a general manner a wastewater treatment process 1according to one embodiment of the invention. In process 1,phosphate-containing wastewater from a wastewater source undergoes afluoride removal step 10, an optional silica removal step 20, aphosphate recovery step 30, and a polishing step 40. The wastewater mayfor example be phosphogypsum pond water. Phosphate recovery step 30yields phosphate compounds in a commercially useful form. Polishing step40 yields treated effluent ready for discharge.

FIG. 2 illustrates another embodiment of the invention following process1 but more specifically exemplifying treatment of phosphogypsum pondwater and recovery of struvite.

Fluoride removal step 10 comprises raising the pH of the wastewater withone or more bases to a desired pH that promotes precipitation ofcontaminants such as fluoride and/or sulphates but not precipitation ofphosphates. In some embodiments the pH may be raised to about pH3.0-4.0. In some embodiments the degree to which the pH is raised mayvary with the composition of the wastewater. Fluoride removal step 10results in relatively dense precipitates that settle well. Theprecipitates may for example be settled and separated in a pond, aclarifier, a separation tank, or the like.

The base used in fluoride removal step 10 may be a calcium-containingbase. The calcium-containing base may be added in an amount such thatthe calcium added to any pre-existing calcium in the wastewater resultsin a concentration of calcium ions sufficient to cause precipitation ofcompounds such as fluorite, calcium fluorosilicate, calcium sulphate,and the like while being insufficient to precipitate significant amountsof calcium phosphate. This may be achieved by adding sufficient calciuminto the wastewater solution at a rate such that the product of thecalcium ion concentration, the concentration of a fluorine-containingionic species and the concentrations of any other components of acalcium salt exceeds the ksp for the calcium salt without being so highas to cause significant precipitation of calcium phosphate. The totalamount of calcium added in step 10 is desirably sufficient to causeprecipitation of the bulk of the fluoride in the wastewater in step 10.For example, a stoichiometric amount of calcium may be introduced duringstep 10 by including the step of measuring the amount of fluoride in thewastewater beforehand. As shown in FIG. 2, the calcium-containing basemay comprise lime, including calcium oxides, carbonates and hydroxides.In other embodiments, the calcium-containing base may be a compoundother than lime.

Alternatively or additionally, one or more calcium-free bases may beadded to raise the pH sufficiently to precipitate the fluoride atfluoride removal step 10. In some embodiments, the calcium-free base maybe selected on the basis of a phosphate compound that is desired to berecovered at phosphate recovery step 30. For example, if the phosphatecompound to be recovered is or comprises struvite, as shown in FIG. 2,suitable calcium-free bases may include magnesium- and/orammonium-containing bases such as magnesium oxide, magnesium hydroxide,ammonium hydroxide, and anhydrous ammonia.

Bases containing magnesium and/or ammonia may be added to simultaneouslyraise pH of the wastewater and increase the concentration of magnesiumand/or ammonia cations to facilitate struvite production in a subsequentstep. For example, magnesium oxide may be used to add magnesium in aquantity sufficient to raise a concentration of magnesium ions to ortoward a concentration desired to later precipitate struvite. Additionof a magnesium-containing base may also assist in removal of fluorideions by promoting precipitation of fluoride as sellaite (MgF₂).

In some embodiments a mixture of two or more calcium-free bases may beused to raise the pH at fluoride removal step 10. Bases may be added ina sequence that accounts for pH-dependent differences in solubility ofthe bases. For example, the base with better dissolution at a lower pHmay be added before the base with lower dissolution at the lower pH. Forexample, if magnesium oxide and ammonium hydroxide are used, thenmagnesium oxide may be added first (because its dissolution is better atlower pH), and then ammonium hydroxide added next to reach the desiredpH for fluoride removal.

Following fluoride removal step 10, process 1 may include a silicaremoval step 20 after measuring the silica concentration of thewastewater. Silica removal may be desirable in some embodiments to avoidsilica gel formation, which may interfere with recovery of phosphatecompounds (e.g. struvite crystallization) at phosphate recovery step 30.In some embodiments, silica may be removed by adding base to hydrolyzethe silica and then allowing the silica to settle. Step 20 mayconveniently be performed in a settling tank or the like. Settled silicamay be removed. In some embodiments, silica may be hydrolyzed by addinga base to adjust the pH to a pH optimal for subsequent phosphaterecovery step 30. In some embodiments, the pH may be at least about 5prior to phosphate recovery step 30.

One or more bases that contain cations (e.g. magnesium and/or ammonia)that will enhance phosphate precipitation at phosphate recovery step 30may be used to raise the pH for silica removal step 20. As shown in FIG.2, the pH may be raised to a pH of about 7.5 in silica removal step 20.In some embodiments a suitable flocculent may be added after silicahydrolysis to further promote aggregation and settling of the silica.

Silica removal step 20 is unnecessary in some embodiments. Since silicagel formation tends to occur only at higher silica concentrations (e.g.Si>100 ppm), embodiments of the invention for processing wastewater withlow silica concentrations may not require the silica removal step. Evenif the silica concentration is high enough for gel formation, thehydraulic retention time of the gel formation is typically on the orderof hours. In contrast, the hydraulic retention time for phosphateprecipitation at phosphate recovery step 30 may be shorter than this.For example, the hydraulic retention time for struvite formation istypically less than 1 hour, although with a high concentration feed thehydraulic retention time may be significantly longer in embodimentsincorporating recirculation as described below. Silica gel formation andthe need for silica removal prior to phosphate recovery step 30 maytherefore be avoided even in some embodiments that process wastewaterwith higher silica concentrations. In some embodiments where silica isnot removed prior to phosphate recovery step 30, silica is hydrolyzedduring phosphorus recovery step 30 and eventually removed downstream.

Silica removal step 20 is followed by phosphate recovery step 30. Asshown in FIG. 2, phosphate may in some embodiments be recovered in theform of struvite, struvite analogs, or other phosphate compounds, forexample according to the methods and apparatus described by Koch et al.in U.S. Pat. No. 7,622,047, incorporated herein by reference. If thewastewater was not treated by silica removal step 20 (and therefore thepH not raised since fluoride removal step 10), then a suitable base maybe added to the wastewater at an initial point of phosphate recoverystep 30 to raise the pH to a desired level for precipitating the desiredphosphate compound(s). As shown in FIG. 2, ammonium hydroxide (or otherammonium and/or magnesium containing base) may for example be used toraise the pH for phosphate recovery step 30 to raise the pH. The pH maybe raised to between pH 7.0 to 8.0, for example to pH 7.5.

Supersaturation conditions for the phosphate compound are maintained torecover desired phosphate compounds during phosphate recovery step 30.Maintaining supersaturation conditions may for example include:maintaining a supersaturation ratio of 2 to 5 for struvite; maintaininga suitable pH, for example by controllably introducing a base and/or airstripping; maintaining phosphate concentration higher thanconcentrations of other components of the phosphate compound; and/orcontrollably introducing compounds comprising at least one of the othercomponents of the desired phosphate compound.

Supersaturation conditions for struvite may be determined in relation tothe struvite solubility product K_(sp) given by:K_(sp)=[Mg²⁺]_(eq)[NH₄ ⁺]_(eq)[PO₄ ³⁻]_(eq)where the activities of the different species (i.e. [Mg²⁺]_(eq), [NH₄⁺]_(eq), and [PO₄ ³⁻]_(eq)) are measured respectively as solublemagnesium, ammonia and orthophosphate at equilibrium. Thesupersaturation ratio (SSR) may be given by:SSR=[Mg²⁺][NH₄ ⁺][PO₄ ³⁻]/K_(sp).Increases in the SSR drive crystallization of struvite.

In the case of struvite recovery, the “other components” mentioned aboveare magnesium and ammonia. During struvite recovery in the embodimentillustrated in FIG. 2, sodium hydroxide is added to control pH andmagnesium chloride is added to control magnesium concentration. Struviterecovery may be “run lean” on magnesium and/or ammonia in someembodiments. Struvite may be recovered in the form of solid pelletsranging in size from 1 to 5 mm in some embodiments.

The methods described in U.S. Pat. No. 7,622,047 may be modified and/orselected to optimize phosphate recovery step 30 in various waysincluding one or more of the following.

-   i. The hydraulic retention time during struvite recovery may be    extended since phosphogypsum pond water tends to have a much higher    concentration of phosphorus compared to municipal wastewater and    pellet formation is rate limiting. This may be achieved, for    example, by increasing a recycling ratio (a proportion of wastewater    that is recycled to wastewater that exits phosphate recovery step    30).-   ii. Ammonia may be added in the form of ammonium hydroxide (or    alternatively as anhydrous ammonia or ammonium chloride plus caustic    for pH adjustment).-   iii. The flow rate of the phosphogypsum pond water feed may be    decreased relative to the flow rate of wastewater being recycled in    phosphate removal step 30 to achieve the desired supersaturation    ratio of 2-5.-   iv. Phosphate may be kept in excess so as to minimize the amounts of    magnesium and ammonium lost in the final effluent.

Following phosphate recovery step 30 the wastewater undergoes polishingstep 40 before being discharged as treated effluent. In someembodiments, polishing step 40 may involve one or more chemicaltreatment steps.

In the embodiment shown in FIG. 2, polishing step 40 includes raisingthe pH of the wastewater from the struvite production step to about pH8-10. This may comprise adding a base such as lime. The elevated pHcauses precipitation of remaining solutes including any phosphate,sulphate, calcium, magnesium, and trace heavy metals. Polishing step 40may also include an ammonia removal step. As shown in FIG. 2, ammoniamay be removed by lowering the pH to about pH 7.0 and subjecting thewastewater to breakpoint chlorination. Other suitable methods ofremoving ammonia include stripping, biological nitrification, biologicaldenitrification, and the like. The ammonia removal step may not beneeded if the struvite recovery step is run lean on ammonia. Afterpolishing step 40, the treated effluent may be discharged.

FIG. 3 illustrates in a general manner a wastewater treatment process100 according to another embodiment of the invention. Fluoride removalstep 110, silica removal step 120, and phosphate recovery step 130 ofprocess 100 may be similar to the corresponding steps of process 1.

FIG. 4 illustrates a further embodiment of the invention followingprocess 100 but more specifically exemplifying treatment ofphosphogypsum pond water and recovery of struvite. Following phosphaterecovery step 130, wastewater in process 100 is polished by membranes150, 160 at polishing step 140. As shown in FIG. 4, prior to membranetreatment the pH of the wastewater may be lowered to about pH 3.0-5.0 toavoid scaling of the membranes, as well as to reduce silica hydrolysisand the risk of membrane fouling. This can be particularly desirable incases where a silica removal step is not performed upstream. Prior tomembrane treatment the wastewater may be pre-filtered to removesuspended solids and to reduce the silt density index. Lowering pHand/or pre-filtration prior to membrane treatment are optional in someembodiments.

First stage membrane 150 may be configured to reject divalent ions (e.g.phosphate, sulphate, magnesium) and let monovalent ions flow through(e.g. sodium, chloride, fluoride, ammonia) to second stage membrane 160.The first stage membrane may for example comprise a reverse osmosis (RO)or nanofiltration (NF) membrane. In some embodiments, the low pHconcentrates (stream A) from first stage membrane 150 may berecirculated to fluoride removal step 110. As shown in FIG. 4,recirculation of stream A diverts divalent ions from discharge andinstead results in: further precipitation of waste components at thefluoride removal step (e.g. removal of sulphate as calcium sulphate atthe fluoride removal step); and further recovery of desired components(e.g. phosphate and the magnesium as struvite) at the phosphate recoverystep.

Second stage membrane 160 may be configured to reject monovalent ions(e.g. sodium, chloride, fluoride). As shown in FIG. 4, the second stagemembrane may comprise a reverse osmosis (RO) membrane. The pH of thepermeate from the first stage membrane may be adjusted up to pH 7.5-8.0before the second stage membrane to promote rejection of fluoride andobtain permeate that can be discharged directly. In some embodiments,the second stage membrane concentrates (stream B) containing mostlymonovalent ions (sodium, chloride, fluoride) may be delivered back tothe wastewater source (e.g. phosphogypsum stack) or to the fluorideremoval step.

An ion exchange (IX) resin bed 170 may be provided to remove ammoniafrom the second stage membrane permeate before discharge as treatedeffluent. Ion exchange regeneration liquid containing the ammonia(stream C) may be recirculated to phosphate recovery step 130 to providepH adjustment and ammonia for recovery of phosphate compounds.

FIG. 5 illustrates in a general manner a wastewater treatment process200 according to another embodiment of the invention. Fluoride removalstep 210, silica removal step 220, and phosphate recovery step 230 ofprocess 200 are similar to the corresponding steps of process 1, andfirst stage membrane 250, second stage membrane 260, and ion exchangeresin bed 270 are similar to the corresponding features of process 100.FIG. 6 illustrates a further embodiment of the invention followingprocess 200 but more specifically exemplifying treatment ofphosphogypsum pond water and recovery of struvite.

Following silica removal step 220, wastewater is directed to first stagemembrane 250. In a manner similar to process 100, the wastewater may beacidified and prefiltered prior to first stage membrane 250. Concentratefrom the first stage membrane is fed to phosphate recovery step 230.This concentrate may contain most of the phosphate at about twice theconcentration compared to the feed for the phosphate recovery steps inprocess 1 and 100. Concentrated phosphate may improve the conditions forthe recovery of phosphate compounds in some cases. The other elements ofthe processes illustrated in FIGS. 5 and 6, including the threerecirculation streams A, B and C, are similar to those described abovein relation to process 100, with the exception that in process 200stream A is generated at the phosphate recovery step instead of asconcentrate of the first stage membrane in process 100.

Recirculation of concentrate streams A, containing for example excessmagnesium, and concentrate stream C, containing for example excessammonia, to upstream steps may result in up to complete recovery ofthese components into recovered phosphate compounds, for example asstruvite.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. For example:

-   i. The methods for ammonia removal described for the ammonia removal    step of process 1 and the ion exchange method for ammonia removal in    processes 100 and 200 are interchangeable.-   ii. Process 1 may be modified to provide recirculation. For example    ammonia recovered at the ammonia removal step may be recirculated to    phosphate recovery step 30.-   iii. The two-stage membrane treatment may be substituted with a    single membrane (e.g. reverse osmosis only) or more than two    membranes (e.g. the two-stage membrane treatment preceded by    microfiltration and/or ultrafiltration membranes).-   iv. Individual features of the various embodiments disclosed herein    may be combined with one another to create further example    embodiments. For example, polishing stage 40 of FIG. 1 may be    expanded to comprise membrane treatment, as shown in FIG. 3 or FIG.    4 or other mechanical polishing processes.-   v. Some embodiments may produce struvite analogs such as iron    ammonium phosphate, wherein corresponding Fe compounds are    substituted for Mg compounds during processing.

The following example provides results of laboratory scale testing ofsome embodiments of the invention.

Example 1

Raw pond water samples were tested in three stages: 1) F removal withCa, 2) pH increase, and 3) struvite precipitation.

In Stage 1, CaCO₃ and Ca(OH)₂ were added to 2 L and 3 L samples of pondwater, mixed for 60 minutes, settled for 30 minutes, then filtered andsupernatants analyzed to evaluate the effect of adding the bases on bothpH and the concentrations of F and PO₄. Ca(OH)₂ was added in both solidand slurried form (results shown for slurried form only). Ca:F molarratios of 0.5 and 0.6 for both reagents were tested, respectivelyrepresenting the stoichiometric amount and a 20% excess amount.

Both CaCO₃ and Ca(OH)₂ reagents raised the pH to between about 2.5-3.5after 1 hour of mixing. CaCO₃ may be preferred in some embodiments. Testresults showed that with CaCO₃ the F removal at 0.6 Ca:F molar ratio waslower than with Ca(OH)₂ at 0.6 Ca:F molar ratio but so were the PO₄ andNH₃ losses. For the remaining stages, CaCO₃ at 0.6 Ca:F molar ratio wasused.

24 hours after completion of the test, more solids had precipitated inthe filtered supernatant, and the SO₄ concentration had decreased alongwith the Ca concentration, indicating gypsum formation.

TABLE 2 Stage 1 results F PO4-P NH3-N SO4-S Ca ID mg/L mg/L mg/L mg/Lmg/L pH Pond water 8800 9088.5 1215 9375 1222 1.17 J1 0.5 CaCO₃ 73908262 1020 2918 4701 3.04 J2 0.6 CaCO₃ 5197 7624 1085 3192 2091 3.23 J30.5 Ca(OH)₂ 5686 7888 1024 3877 2831 3.49 J4 0.6 Ca(OH)₂ 4409 7517 10227096 2616 2.66 J1 % removal 16.0%  9.1% 16.0% 68.9% J2 % removal 40.9%16.1% 10.7% 66.0% J3 % removal 35.4% 13.2% 15.7% 58.6% J4 % removal49.9% 17.3% 15.9% 24.3%

In Stage 2, Mg(OH)₂ was added (in slurried 40 wt % form) to the 500 mLand 1250 mL samples of Stage 1 supernatant in Mg:P molar ratios of 0.8,0.9, and 1.0, to raise the pH of the solution nearer the pH required forstruvite precipitation and also to put Mg ions in solution. Also, MgCl₂was added in 1.0 Mg:P ratio to compare the effects of adding a non-basicMg source at this stage.

The Mg compounds were added immediately after the completion of arepeated Stage 1 test, to prevent Ca loss through gypsum precipitation.The solutions were mixed for 60 minutes and settled for 15 minutes.

The Mg(OH)₂ raised the pH to 4.5-5.5, and caused nearly complete removal(>90%) of both Ca and F. A substantial amount of PO₄ was also removed,but the quantity remaining was still high and sufficient for struviteproduction downstream. A substantial amount of the added Mg was alsoremoved in this stage. The MgCl₂ did not raise the pH but slightlylowered it, and had very little effect on either F removal or P loss.Increasing the Mg:P molar ratio from 0.8 to 1.0 increased F removal byonly 2.8% but increased PO₄ losses by 11.5%. 0.8 Mg:P was selected foruse in Stage 3.

TABLE 3 Stage 2 results F PO4-P NH3-N SO4-S Ca Mg ID mg/L mg/L mg/L mg/Lmg/L mg/L pH Pond 8800 9089 1215 9375 1222 551 1.17 water A 0.6 60207904 1152 7656 5993 502 3.06 CaCO₃ J1 0.8 483 5421 1135 7656 536 22084.47 Mg(OH)2 J2 0.9 370 4993 1123 7656 366 2234 4.86 Mg(OH)2 J3 1.0 3124513 1068 7656 230 2242 5.27 Mg(OH)2 J4 1.0 5430 7583 417 8975 59956204.7 2.79 MgCl₂ A % 31.6% 13.0%  5.2% 18.3% 8.9% removal J1 % 92.0%31.4%  1.5%   0% 91.0% removal J2 % 93.9% 36.8%  2.5%   0% 93.9% removalJ3 % 94.8% 42.9%  7.3%   0% 96.2% removal J4 %  3.2%  4.7%  6.1%   0%  0% removal J1 overall 94.5% 40.4%  6.6% 18.3% removal J2 overall 95.8%45.1%  7.6% 18.3% removal J3 overall 96.5% 50.3% 12.1% 18.3% removal J4overall 38.3% 16.6% 65.7%  4.3% removal

In Stage 3, NH₄OH was added to 500 mL samples of Stage 2 supernatant inN:P molar ratios of 0.8 and 1.0, then NaOH was used to raise the pHabove 7.0. As the Mg:P ratio was approximately 0.5:1 due to the Mg lossin Stage 2, a P recovery of near 50% would be expected if the P wereprimarily forming struvite. The Mg was 99% removed, showing that thereaction proceeded as far as it could given the Mg limits, and the Premoval was near 58%. Struvite precipitation in wastewater is Mg limitedas well, and MgCl₂ or other sources of soluble Mg can be added.

TABLE 4 Stage 3 results: F PO4-P NH3-N SO4-S Ca Mg Final ID mg/L mg/Lmg/L mg/L mg/L mg/L pH pH Pond water 8800 9088.5 1215  9375 1222 551.11.17 J1 0.6 CaCO₃ 5608 7957  444* 6099 5450 496.94 3.13 J2 0.8 Mg(OH)₂477 5194 412 8756 554.9 2080.6 4.62 J3-1 0.8 NH₄OH 277 2190 558 894867.6 13.158 5.52 7.40 J3-2 1.0 NH₄OH 267 2150 788 6729 65.7 13.0 5.847.36 J1 % removal 36.3% 12.4% 63.5% 34.9%  9.8% J2 % removal 91.5% 34.7% 7.2%   0% 89.8% J3-1 % removal 41.9% 57.8%   0% 87.8% 99.4% J3-2 %removal 44.0% 58.6% 23.1% 88.2% 99.4% J3-1 overall removal 96.9% 75.9%J3-2 overall removal 97.0% 76.3% * This number (and the ones below it)are anomalous, and should be similar to the numbers obtained at the endof the Stage 2 test wherein ammonia levels were about 1100 mg/L.

Example 2

An overall pH test was also conducted. 250 mL of the pond water samplewas placed in a beaker. The blade of an overhead mixer was placed in thesample and rotated at 70 rpm. 6.95 g CaCO₃ was added to obtain a 0.6Ca:F ratio. The pH was monitored every 15 minutes. The pH was recordedat 60 minutes.

3.95 g Mg(OH)₂ slurried in 5.4 g water was added to obtain a 1:1 Mg:Pratio, based on previous jar test results from Example 1. The pH wasmonitored every 15 minutes. The pH was recorded at 60 minutes.

2.16 g dry basis/7.17 g 30 wt % NH₄OH was slowly added to obtain a 1:1NH₄OH:P ratio based on P after the CaCO₃ precipitation.

TABLE 5 pH test results ppt with CaCo3 ppt with Mg(OH)2 ppt with NH4OHTime Time Time Cum. NH4OH (min.) pH (min.) pH (min.) pH dry basis added 0 1.8  0 3.09  0 5.17 0.54 15 3.2 15 4.31  10 5.85 1.08 30 3.2 30 4.77 20 7.61 45 3.2 45 5.03  60 7.3  2.16 60 3.1 60 5.17 120 9.49

REFERENCES

-   Kennedy, G. A., Soroczak, M. M. and Clayton, J. D., “Chemistry of    Gypsum Pond Systems”, Florida Institute of Phosphate Research (FIPR)    Project #85-05-025R, 1991.-   Perpich, B, Jr., Soule, C., Zamani, S. Timchak, L., Uebelhoer, G.,    Nagghappan, L. and Helwick, R., “Mobile Wastewater Treatment Helps    Remediate Concentrated Acidic Process Water at Fertilizer Plant”,    Florida Water Resources Journal, July 2005.

The invention claimed is:
 1. A method for treating, and recoveringphosphate compounds from, wastewater, the method comprising: (a)measuring, precipitating and removing fluoride from the wastewater byraising the pH of the wastewater by adding a calcium-containing basewith a stoichiometric amount of calcium to precipitate the fluoride,wherein the pH does not promote precipitation of phosphates, and thenfurther raising the pH of the wastewater by adding one or morecalcium-free bases; (b) recovering struvite from the wastewater fromwhich fluoride has been removed by maintaining supersaturationconditions for the struvite; and (c) polishing the wastewater whereinstep (c) comprises subjecting the wastewater from step (b) to atwo-stage membrane treatment comprising: (i) a first membrane treatmentto obtain a first concentrate comprising divalent ions and a firstpermeate comprising monovalent ions; and (ii) a second membranetreatment for the first permeate to obtain a second concentratecomprising monovalent ions and a second permeate comprising effluent. 2.A method according to claim 1 wherein the first concentrate isrecirculated to step (a).
 3. A method according to claim 1 wherein thefirst membrane treatment comprises nanofiltration.
 4. A method accordingto claim 1 wherein the second membrane treatment comprises reverseosmosis.
 5. A method according to claim 1 comprising lowering the pH toabout pH 3 to 5 prior to the two-stage membrane treatment.
 6. A methodaccording to claim 1 comprising removing suspended solids by filtrationprior to the two-stage membrane treatment.
 7. A method according toclaim 1 comprising removing ammonia from the second permeate.
 8. Amethod according to claim 7 wherein removing ammonia comprisessubjecting the second permeate to ion exchange.
 9. A method according toclaim 8 wherein ammonia-containing liquid of the ion exchange isrecirculated to step (b).
 10. A method according to claim 1, whereinprior to step (b) the wastewater is subjected to a first membranetreatment to obtain a first concentrate comprising divalent ions and afirst permeate comprising monovalent ions, wherein the first concentratedefines feed for step (b).
 11. A method according to claim 10 whereinwastewater from step (b) is recirculated to step (a).
 12. A methodaccording to claim 10 wherein the first permeate is subjected to asecond membrane treatment to obtain a second concentrate comprisingmonovalent ions and a second permeate comprising effluent.
 13. A methodaccording to claim 12 wherein the second membrane treatment comprisesreverse osmosis.
 14. A method according to claim 10 wherein the firstmembrane treatment comprises nanofiltration.
 15. A method according toclaim 10 comprising lowering the pH to about pH 3 to 5 prior to thefirst membrane treatment.
 16. A method according to claim 10 comprisingremoving suspended solids by filtration prior to the first membranetreatment.
 17. A method according to claim 10 comprising removingammonia from the second permeate.
 18. A method according to claim 17wherein removing ammonia comprises subjecting the second permeate to ionexchange.
 19. A method according to claim 18 wherein ammonia-containingliquid of the ion exchange is recirculated to step (b).